French patent application No. 2 463 955 describes a localization device using a rectangular resistive plate provided with contacts disposed along its sides. The electrical contacts situated along a single side are connected to a corresponding common summing line via respective resistances. The summing lines associated with two opposite sides are connected to the inputs of a differential circuit which delivers an analog signal representative of one of the two co-ordinates of the point at which the electrical charge was injected which gave rise to the currents picked-up by the various contacts. The summing lines associated with the other two sides are connected to the inputs of another differential circuit which delivers an analog signal representative of the other co-ordinate of the point.
A special selection of resistance values connecting the contact points to the summing lines makes it possible to minimize distortion but does not avoid it altogether.
Japanese patent application published under the No. 58-99889 also describes a localization device using a rectangular resistive plate provided with electrical contacts connected to summing lines via particular resistances. In order to reduce distortion, resistances are inserted between the electrical contacts situated on two adjacent sides of the plate, in particular between contacts close to the corners of the plate.
Here again, although distortion is reduced, it nevertheless remains significant.
Another technique for eliminating distortion consists in converting the amplitudes of the currents picked up by the electrodes into digital form (optionally after summation and normalization) and then in using computer means to process the digital values obtained in order to correct for the distortion. It is relatively expensive to use computer means in this way and, in some applications, the delay resulting from processing time can be particularly troublesome. Further, distortion varies over time and with environmental conditions. The function for correcting a particular nonlinearity at a given instant need not be valid later on.
In order to reduce nonlinearity, proposals have also been made to use specially-shaped electrodes. However nonlinearity is not completely eliminated. Computer processing is still required to correct distortion with the same drawbacks as mentioned above together with the additional difficulty of making the specially-shaped electrodes.
The present invention seeks to provide a localization device which supplies signals representative of an undistorted image without requiring cmputer processing or particularly complicated electrodes.